Inverters using controlled semiconductor rectifiers

ABSTRACT

A parallel D.C. to A.C. inverter uses triggered silicon controlled rectifiers (SCR&#39;&#39;s) in a power current flow circuit especially adapted to supply variable frequency currents to an induction motor load over a wide range of frequency, voltage and load power. A bifilar choke is used to prolong the discharge time of the commutating capacitor and to prevent high peak currents to the load during commutation. A first pair of diodes prevents discharge of the capacitor prior to commutation. A second pair of diodes provides alternate feedback paths for the return of energy stored in the magnetic circuit to the power supply immediately after commutation.

United States Patent Wurstet a1.

INVERTERS USING CONTROLLED SEMICONDUCTOR RECTIFIERS 3,317,816 5/1967 Wilting.. ..321/45 [151 3,702,432 I451 TNov. 7,1972

3,417,315-12/1968 Corey 32 1/45 Primary Examiner--William M. snoop; Jr.

Attqrney -Marshall .l. Breen and Chester A. Williams, Jr. t

s7 ABSTRACT A parallel DC. to A.C. inverter uses triggered silicon controlled rectifiers (SCRs) in a power current flow circuit especially adapted to supply variable frequency currents to an induction motor load over a wide range of frequency, voltage and load power. A bifilar choke is used to prolong the discharge'time of the commutating capacitor and to prevent high peak currents to the load during commutation. A first pair of diodes prevents discharge of the capacitor prior to commutation. A second pair of diodes provides alternate feed back paths forthe return of energy stored in the magnetic circuit to the power supply immediately after commutation.

1 Claim, 2 Drawing Figures syn 11 IO l6 v1.47 vI8 le Fi .1 (PRIOR ART) I QB IRT v 24- r25 C l7 l8 LiI kg F Fig. 2

INVENTORS John W. Wurst Robert M. Konen WITNESS= BY ATTORNEY INVERTERS USINGCONTROLLED SEMICONDUCTOR n nc'rmrsas BACKGROUND-F THE nvsrmou .'Prior an inverter circuits using SCRs are knownfor providing an A.C. output to induction motor .loads from a D.C. supply voltage. These have, in general,

provided a single frequency output, or where variable outp'utffrequency has been obtained, the. range of frequency variation has'b'een severely limited for stable operation. The problem isrelated' to-theability of the commutating capacitor to holdavoltage charge long enough to maintain previously conducting SCR'in a reversed biased condition long enough to enable it to recover its forward blocking capability. This is particularly difficult at the highe'r'output frequencies and for loads which vary considerably in size and power factor.

. It is therefore a primary objectof this invention to provide a reliable alternating drivingvoltagefrom a D.C. supply voltage, which alternating voltage shall'be' substantially free from adverse effects of variable load dissipation and power factor-over a wide range of operating frequencies,

' choke performs two importantfunctions. Because the magnetic-circuit of thechoke is common to both windings, it smooths the flowofburrehtfromthe D.C.

source, preventing'high peak currents from flowing 2 The load winding 12 is shown, for simplicity, as the primarywinding of a transformer 19 having itssecondary connected to a'load'20'. However, it will be understood that the winding 12 may also itself be a phase winding of an induction motor. or that the loadmay be such a motor phase winding. In any case, it willbe understood that winding 12' may connect with A.C.

loads of varying size and power factor.

. The operation of the circuit of FIG. 1 is as follows: Current from the D.C. power source at terminal 13 enters the centertap of the A .C. load winding 12. It is permitted to flow alternately through each half of wind ing 12 depending on which SCR 10. or 1-1) has been.

triggered into conduction. Reversal of current -flow,'or commutation, occurs when the non-conducting SCR is triggered into the conducting state. The charge on capacitor l6'holds the previously conducting SCR in a reversed. biased condition until that SCR recovers its forward blocking condition, I

The problem with this circuit arises from the variationof the charge on the capacitor 16. When one SCR is conducting, the D.C. supply voltage. (minus about '1 during commutation in bothhalves of the loadwindin'g.

Secondly, it .is them'ajor'factor in controlling the charge on. the commutating capacitor. Duringthe first DESCRIPTIGN OF THE INVENTION Referring to FIG. 1, a basic prior. art circuit is shown comprising two SCRs l0 and 11, an A.C. load winding 12 having a-centertap connected to a terminal 13 which is the positive side of a D.C. supply source (not shown). The SCRs 10 and 11 have their. anodes connected respectively, to opposite ends of the A.C. load winding.

12 and their cathodes connected to a common line 14 connected toa terminal 15 which is the negativeside of the D.C. power supply.

A commutating capacitor 16 is'connected between the two anodes and triggering voltages are applied to g the gates-l7 and 18 on an alternative basis to cause SCRs l0 and 11 to conduct sequentially as is well known in'this art. The frequency with which the triggen'ng voltages are applied determines thefr'equency of the A.C. to the load winding 12.

volt drop across the SCR).appears between the centertap andone end of thewinding 12. By transformer action, a similar EMF is induced in the other half of the winding 12 such that the capacitor sees a voltage equal approximately to twice theisupply voltage. In practice, this voltage varies with the frequency and load conditions so that under some circumstances insufficient energy isstored-in the capacitor to insure commutation resulting in failure of the. A.C. output.

To examine this process more particularly, assumev that SCR 10 is conducting. The capacitor 16 then charges to thepolarity shown in FIG. 1 with its positive terminal connected to the anode of SCR 11. When the SCR 11 is now triggered into conduction, SCR 10 sees the negative capacitor terminal at its anode and the positive terminal essentially at its cathode. Thus, SCR 10 is back-biased and turns off. If this capacitor voltage remainsznegative for longer than the SCR- recovery time, which'is typically 5 to 15 microseconds, the'SCR 10 will not conduct again when the capacitor charge is consumed or reversed. However, the capacitor 16 can discharge through the winding 12 and through the power supply'and in fact almost all of the discharge current does pass through the winding 12. If the load resistance 20 is small, or if the required power'output of a motor load is high, which condition causes a low equivalent resistance to appear in the winding 12, then the'capacitor discharges more rapidly. Consequently the reverse bias appears on the SCR (to be turned off) for a-shorter period of time. In the limiting condition, this reverse bias is not sustained for as long as the required recovery time. Then both SCR l0 and SCR 11 remain in conduction, currentconsumptionis high, and

no alternating voltage output is produced.

With this failure process of the prior art circuit in Certain elements have been added which will now be I described in detail:

' Abifilar choke 21 comprises two identical mutually insulatedwindings 22' and 23 on'a common magnetic core. The windings are as closely coupled as possible so that the reactive voltage generated in one winding is,

reflected immediately in the other winding. Winding 2 2 'is'connected in series with one'end of load winding .12

and a diode 24 to the junction of the anode of SCR10 with-the capacitor 16. Winding 23 is connected in series with theopposite end of load winding 12 and a diode 25 to the junction of the anode of SCR with the capacitor 16. The sense of the windings 22 and 23 is winding 12 decreases to zero and then builds in the normal direction through SCR 10..This current I represents the reactive current that must be returne the power supply from motor loads where the power such as to produce the same instantaneous polarities at I between one end of the load winding 1-2 and the negative terminal 15 ofthe DJC. supply. A second feedback diode 27 of the polarity shown is connected between the opposite end of the load winding l2 and the negative terminal 15 of the DC. supply.

The operation of the circuit of FIG. 2 is as follows:

Assume that capacitor 16 is charged with the polarity as indicated in FIG. 2because SCR '10 is conducting. When'a trigger pulse is applied to the gate 18, SCR 11 will go into a conducting state. Capacitor discharge current indicated by I will flow through SCR 11, diode 26, choke winding 22, diode 24 and into the negative terminal of capacitor 16. SCR 10 is back-biased by this action and turns off. Choke winding 22 develops a voltage which opposes this currentchange and so prolongs the discharging time of the capacitor 16. Ultimately, the capacitor charge will approach zero at which time the voltage induced inwinding 22 will reverse its polarity in an effort to keep the. current flowing. In effect, energy stored in the capacitor 16 has been transferred to magnetic energystored in the choke 21. This energy then returns to the capacitor 16 but charging it in such a direction that the polarity of its terminals is reversed from, that shown. Diode 24' prevents its discharge and a peakcharge isretained for the next commutating operation. This charge may be stored at a higher voltage than would be the case without the choke 21 because of series resonance effects. Depending on the characteristics of the choke 21, this voltage may be typically 2 to 10 times the peak voltage appearing across the terminals of load winding 12 and is subcurrent l in the same direction through the left hand i half of winding 12 asshown. This current (I flows towards the positive terminal 13 of the power supply,

then reenters through the negative terminal 15 through diode 26 and back to the motor winding 12., As the force driving this current becomes less than the potential of the power supply, the current flow 1,; through the factor is not unity; It will be noted that this flow ofreactive current has no direct effect on the charging of capacitor 16.

Clearly, from the above, it will be evident that the choke 21 provides a charge and discharge pathfor the capacitor 16 which does not require commutation current to flow through the load winding 12 and therefore the commutation is essentially independent of load ef-' fects. Thus the circuit can be designed for good com-- mutation with the assurance that load changes will not cause deterioration thereof. Since very little commutation power is dissipatedin theload winding,-.thecommutation process is more efficient and a smaller commutating capacitor cuit s.

Further, a flow path for the reactive load currents due to low power factor loads is provided which is substantially isolated from any effect on the charge and discharge of the capacitor 16. Thus, changes in the load. power factor can be tolerated without adverse effect on the commutation process.

may be usedthan inprior art cir- By the attributes of this invention an inverter circuit 2 has been provided which has the following advantages:

1. Reliable commutation over a wide range of frequency, load'and power factor.

2. Smaller commutating capacitor than otherwise required.

3. Less commutation power dissipation in the load winding.

4. Nearly constant SCR hold-ofi' time as operating conditions vary. While the invention has been described in relation to a specific embodiment, many modifications and variations thereof will be readily apparent to thoseskilled in the art without departing from-the inventive concept, the scope of which is set forth in the appended claims.

Havingthus set forth the nature of the invention, what is claimed herein is:

1. An inverter circuit adapted to be connected to a I D.C. supply source having positive and negative terminals, comprising: an A.C. load winding having a centertap connected to said positive terminal, a pair of gate controlled rectifiers each. having an anode, a

cathode and a gate, means connecting the cathodes together and to said negative terminal, a non-saturable choke having two mutually-insulated windings closely negative terminal and a respective opposite end of the load winding and of a polarity to conduct currentinto the load winding. 

1. An inverter circuit adapted to be connected to a D.C. supply source having positive and negative terminals, comprising: an A.C. load winding having a centertap connected to said positive terminal, a pair of gate controlled rectifiers each having an anode, a cathode and a gate, means connecting the cathodes together and to said negative terminal, a non-saturable choke having two mutually-insulated windings closely coupled on a common magnetic core, means connecting each of said choke windings in series between a respective opposite end of the load winding and the anode of a respective controlled rectifier, each of said series connections including a diode of such polarity as to conduct current in the same direction as the controlled rectifier, a commutating capacitor connected between the anodes of said controlled rectifiers, and a pair of feedback diodes, each connected between the negative terminal and a respective opposite end of the load winding and of a polarity to conduct current into the load winding. 